Study of the lagoons of Petriky (Madagascar) before the ilmenite mining exploitation - Point zero I Physicochemical study
Résumé
An open-air exploitation of an Ilmenite mine has been set near lagoons of Petriky (South-East of Madagascar). The lagoons are a set of three contiguous lagoons of average size ranging from 5 to 10 km². The first lagoon Ambinanibe collect in the north the waters of the Ifaho River and is temporary connected to the ocean with a channel. The central lagoon, Andranasy, has a permanent connection with the lagoon of Ambinanibe and this late has a temporary connection with the third lagoon of Andrantoloharano. The aim of this work is to study the physicochemical characteristics of the lagoons before the disturbances caused by the mine. A several years temporal monitoring, allowed us to assess the biogeochemical state and the ecological functioning of the water column. A synthesis of all the results allows us to set up an assessment on the lagoons functioning before their deterioration owing to anthropogenic activities. This will constitute a witness of what existed before drastic perturbations and will enable them to check whether the post exploitation rehabilitation provided is successful. The three lagoons of Petriky are brackish. The oxygen values do not vary significantly between the lagoons 7.14 ± 0.11 mg/l. Anoxia does not exist there. The major enrichments come from the catchment basin of Ifaho. Throughout the year, the lagoons are never nutrients depleted. The enrichments occur after the river floods. The enrichment in nitrate begins in January through the lagoon of Ambinanibe and progressively reaches the other lagoons with a delay of a month. After the floods, the peaks of nitrate concentration are huge and the variation of concentration between minima and maxima vary from 200 to 400 according to the lagoons. The concentrations of the surface water in nitrate vary between 1 0.4 and 158.5 µmol/l in Ambinanibe. The silicate enrichment follows this schema and starts in November in Ambinanibe with a time lag from the first lagoon to the third.